class Solution {    public int numDecodings(String s) {        if(s.length()==0){            return 0;        }        int[] dp = new int[s.length()];        dp[0] = s.charAt(0)=='0'?0:1;        if(s.length()==1){            return dp[0];        }        int k = s.charAt(0) > '0' && s.charAt(1) > '0'? 1:0;        dp[1] = k + (s.charAt(0) == '1' || s.charAt(0) == '2' && s.charAt(1) <= '6' ? 1:0);        for (int i = 2; i < dp.length; i++) {            if(s.charAt(i)!='0'){                dp[i] += dp[i-1];            }            if(s.charAt(i-1)=='1'||s.charAt(i-1)=='2'&&s.charAt(i)<='6'){                dp[i] += dp[i-2];            }        }        return dp[s.length()-1];    }}

 

补充一个python的实现：

1 class Solution: 2     def numDecodings(self, s: str) -> int: 3         if not s: 4             return 0 5         dp = [0 for x in range(len(s) + 1)] 6         dp[0] = 1 7         dp[1] = 1 if 0 < int(s[0]) <= 9 else 0 8  9         for i in range(2, len(s) + 1):10             if 0 < int(s[i-1:i]) <= 9:11                 dp[i] += dp[i - 1]12             if s[i-2:i][0] != '0' and int(s[i-2:i]) <= 26:13                 dp[i] += dp[i - 2]14         return dp[len(s)]

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